Ion channel glutamate receptors are ligand-gated transmembrane proteins that can be activated by the binding of glutamate, the principal excitatory neurotransmitter in the brain. Ionotropic glutamate receptors (iGluRs) are, therefore, the major excitatory neurotransmitter receptor proteins in the mammalian brain. As such, these receptors play special roles in brain activities, such as memory and learning, and have been implicated in a variety of neurological diseases, such as post-stroke cellular lesion and amyotrophic lateral sclerosis [Dingledine et al., 1999; Heath and Shaw 2002].
When glutamate, released from a presynaptic neuron, binds to a postsynaptic glutamate receptor, the receptor rapidly changes its conformation and transiently forms an open ion channel, thus resulting in a change of the postsynaptic membrane potential. A postsynaptic potential of sufficient strength triggers an action potential, which will in turn propagate the initial nerve impulse. The major function of iGluRs is to mediate fast synaptic neurotransmission underlying the basic activities of the brain, such as memory and learning. Excessive activation of ionotropic glutamate receptors, particularly the α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) subtype, is known to induce calcium-dependent excitotoxicity. Excitotoxicity has been considered as a general pathogenic mechanism underlying a number of neurological disorders such as amyotrophic lateral sclerosis (ALS), stroke, Alzheimer's disease and Parkinson's syndrome.
Using inhibitors to dampen the excessive activity of these receptors may serve as a treatment for neurological disorders such as ALS or Huntington's disease. To date, Riluzole, an inhibitor of presynaptic glutamate release, is the only drug that benefits the survival of ALS patients. Currently, the majority of AMPA receptor inhibitors are those synthesized by organic chemistry, and many of them show cross activity to kainate receptors, another subtype of iGluRs. The cross activity is not desirable, because the AMPA and kainate receptors have functional differences. Furthermore, the majority of AMPA receptor inhibitors have poor water solubility. In addition, there is a lack of an assay of inhibitor-receptor interactions within the microsecond (μs) to millisecond (ms) time domain. This is because an AMPA receptor opens its channel in the μs time scale and desensitizes within a few ms in the continued presence of glutamate. Consequently, the potency of all AMPA receptor inhibitors has been determined only with the desensitized receptors. These deficiencies have significantly hampered drug development.
Because proteins are generally dynamic and adapt a specific conformation for function, using molecular agents that bind selectively to a specific protein conformation among its conformational repertoire is thus a powerful means to exert a tighter molecular recognition to more effectively regulate the existing function of that protein, and to even engineer a new protein function. For instance, small chemical compounds have been found to stabilize a conformation for some apoptotic procaspases to induce autoproteolytic activation of these proenzymes. Catalytic antibodies have been created, based on transition-state structural analogs, to accelerate chemical reactions by stabilizing their rate-determining transition states along reaction pathways. Developing inhibitors to control excessive receptor activity has been a long pursued therapeutic strategy for a potential treatment of these neurological disorders and diseases.
What is needed, therefore, is an AMPA glutamate receptor inhibitor that is characterized by a high affinity for its target, preferably in the nanomolar range, specificity targeting the glutamate receptor, excellent water solubility and relevance of its inhibitory properties to the functional forms of the receptor rather than the desensitized receptor forms.